1. Field of the Invention
The present invention relates to a microphone output transmission circuit, and particularly relates to such a circuit which is suitable for use with a capacitive, or condenser microphone of the electret type or the bias type.
2. Brief Description of the Prior Art
The bias-type condenser microphone requires a DC bias voltage applied between its diaphragm and its fixed electrode. The electret condenser microphone, while not needing a bias voltage, still employs an FET pre-amplifier which, in turn requires a power source. Therefore, in either case it is necessary for the transmission cable for the output signal of a condenser microphone to provide both signal lines and power lines. It is conventional to arrange the signal lines and power lines in common in order to minimize the number of conductors required.
One conventional arrangement of a transmission circuit for a capacitive microphone generally employs an FET preamplifier coupled to the capacitive microphone and to the primary winding of an audio transformer. The secondary winding of the transformer provides the audio signal as a balanced signal to a balanced pair of conductors. A phantom powering system can be employed in which DC power is superimposed on both balanced conductors, and is derived at a center tap of the transformer secondary to power the FET preamplifier. A ground return is then provided, for example, by a braided shield surrounding the balanced conductors. Because this arrangement requires transformers for signal transmission, the signal quality is easily degraded. More particularly, the frequency response of a transformer is limited, and is further degraded by the presence of a DC current in the secondary windings.
An alternative conventional arrangement avoids the problem caused by DC current in the windings by employing a DC shunt formed of two equal-value resistors connected in series between the secondary terminals, and by deriving the DC power from the junction of the resistors, rather than from the secondary winding center tap. However, in this arrangement the resistors also shunt the signal as well as the DC power, which can result is unacceptable signal-power attenuation.